Many metallic objects are coated, either to render them more resistant
to the action of external agencies or to enhance their appearance, with
more or less thin layers of other metals or of oxides. These coatings are
easy to characterise by the tests given below.
1. Technical Test. — The surface of the object is rubbed repeatedly
with a small piece of very fine glass paper (No. ooo) so as to concentrate any
gilding at one point of the glass paper. This point is then treated with a
drop of cone, nitric and one or two drops of cone, hydrochloric acid and
warmed gently over a very small flame until the metal is dissolved. The
solution is then washed into a test-tube with 1-2 c.c. of water, the liquid
being filtered if turbid and heated with either an equal volume of fresh
sulphur dioxide solution or a few drops of fresh stannous chloride solution.
In presence of gold, a violet red, varying coloration is observed owing to
the formation of purple of Cassius
2. Test for Small Objects. — The sample, or part of it, or a number of
small pieces, according to circumstances, are heated on the water-bath with
nitric acid diluted with an equal volume of water. When the attack of the
common metal is complete, the liquid is filtered through a small filter and
the residue washed thoroughly with hot water. The filter is then incinerated
in a porcelain dish or crucible and the ash treated with 2 drops of cone.
hydrochloric acid and i drop of cone, nitric acid and evaporated to dryness
on a water-bath until the excess of acid is entirely expelled. When cold,
the residue is taken Up in about 2 c.c. of distilled water and filtered, the
filtrate being heated to boiling with an equal volume of sulphur dioxide
solution or of a saturated oxalic acid solution or a few drops of stannous
chloride solution. If gold is present, the characteristic violet- red coloration
of purple of Cassius is observed.
3. Test for Large Objects. — After removal of any organic matter,
the surface is scraped with a penknife and the scrapings submitted to the
1. Technical Test. — The article to be tested, freed from grease,1 is
touched with a drop of cone, nitric acid and the latter absorbed by a strip
of filter-paper. The spot is then treated with a drop of formaldehyde solution
(commercial formalin) and a drop of 20% sodium hydroxide solution. In
presence of silver, a blackish spot of reduced silver forms either immediately
or after some time.
1 Sometimes objects are covered with a varnish which has nitre- or acetyl-cellulose
as its base and can be removed only mechanically.f silver.